1. Field of the Invention
The present invention relates to power systems in general and in particular to power systems that include stand-by batteries and connect/disconnect devices.
2. Prior Art
Most electrical equipment is powered from a power supply connected to main power lines. This equipment is subjected to intermittent loss of power caused by someone unplugging the power supply cord from the power outlet or during service interruption or power outages. There are several types of electrical equipment and appliances which require continuous power.
An acceptable way of providing continuous power is to include a battery in the power supply system. The battery is charged when the main power is operational. However, in the event of a power outage, the power is supplied from the battery. When a battery is used as a stand-by power source, one has to ensure that the battery is not deep discharged (i.e., discharged below a predetermined voltage level). If one deep discharges a battery, the life of the battery is greatly reduced.
It is known in the prior art to connect the battery via a switching mechanism to a load. In the event the battery is discharged to a predetermined voltage level, the switching mechanism is activated to disconnect the load and prevent the battery from deep discharging. Prior art examples of stand-by power supplies with switching mechanisms which disconnect the battery are set forth in U.S. Pat. No. 4,704,542, JP55-76578(A), Appl. No. 53-150695 entitled "Over Discharge Preventing Circuit for Battery" to Anritsu Denki et al and German patent DT 2732-794. Still other prior art circuits and techniques use a positive feedback resistor connected between the load and a trigger transistor to connect/disconnect the battery.
Even though the prior art circuits work well for their intended purposes, they do not switch instantaneously. As a result, even after switching, the battery still continues to discharge through circuitry load. The continued discharge below the threshold voltage level tends to damage and reduce the life of the battery. Also, the period of indecision occurring between activating of the switching circuit and when it finally switches, causes electrical noise in the load. There are certain types of load (e.g., computer systems) which are very sensitive to electrical voltage noise. Because of the noise, the prior art switching systems are not well suited for noise sensitive loads.